The proposed experiments will provide new information concerning sarcomere mechanics and the influence of interfilamentary forces in striated muscle contraction. The experiments will use recently developed techniques which allow direct control of the intracellular ionic environment and the sarcomere and interfilamentary spacings of cardiac and skeletal muscle cells. We propose to examine systematically the separate and interactive relationships between force, sarcomere velocity, sarcomere length, interfilamentary spacing (fiber diameter) and calcium in frog semitendinosus fibers and guinea pig ventricular tissues. A statistical thermodynamic approach will be used to model muscle forces. The experiments should provide new insight into the primary factors involved in modulating sarcomere force and velocity in heart and skeletal muscle, particularly resistive or restoring forces dependent on sarcomere length and interfilamentary spacing and ionic strength.